best available cop



June 14, 1927. 1,632,601

H. M. HOBART INDUCTION MOTOR Filed Oct. 4, 1924 Inventor:

Henry M. Hobart,

' HisAttorney.

Patented June 14, 1927.

UNITED PATENT OFFICE.

HENRY M. ROI-BART, F; sc ENnc'rADm iiEw YORKQASSIGNOR T0 GENERAL ELECTRIC COMPANY,- A CORPORATION OF NEW YORK.

f iNDUoTIoN: Moron.

Application filed October 4, 119124. $erial No. 741,743.

My invention relates to induction motors, and more particularly to the secondary windings of such motors.

My invention relates to an improvement in the type of secordary induction motor windings described in United States Patent 1,188,182, Kierstcad, June 20, 1916. The winding described in said patent is made'up of two or more. relatively deep conductor bars pei'fslot of equal depth arrangedone above the other and connected so that equal secondary currents will 'fiowin all of such conductors. By this arrangement an increased starting torque is obtained tiecause of the increased eddy current loss,produce d in theupper conductor or conductors'dueto the flux cutting said conductors set up largely by the current flowing in a lower conductor or conductors. V I

According to the Kierstead patent the various conductors in a slot aretdes'ignedto be of equal depth and cross sectional area. I ,have discovered that in a two-bar perfslot winding of this type it isonlynecessary to have the upper conductor off substantial depth and that the lower conductor may be made relatively thin radially without sacrificing the beneficial effects o fjthisftype of Windingandthat by such an .arnangementfa 0 more economical and desirable distribution of copper in the secondary winding obtain'ed. f

, According tomy;invention l inalieth Lupper conductor in the slot relatively deep and the lower; conductor relatively-- shallow fan'd arrange, the connections such that thetfsecondary current in both conductor sections per' slot is'the; same. Since the eddyg' ciurrents exist for the ,fmostpart in the upper conductor. and are greater ,t he greater! its depth, important that l the 'greatest practicable part of th'e ftota l' available depth should be. allotted, to the upper conductor.

. The features: of my: iityeiltio l. Whitghflre believed to, benovel and .paten tablefwillibe flow in both sections; Fig. 2 shows a perillustration of the current and flint dist-ribu tions of the conductor sections at different frequencies. f

Eddycurrents in the secondary windings iof an induction motor .at starting produce high lossesin this winding and thereby increase-the starting 'torque "of "'t'he motor. Since the eddy currentlosses are a direct function of the frequency, itwill'beappan ent' that rents-produce the secondary windings of an inductionmotor high 'losssvyhen the secondary currents" are of high frequenc'yjas when ,themotor isstarting or when the :slip' is large.

is lup" to its fullfs' peed however'," thefeddy currents. are nearly zerosince I the}; frequency of the seeondary c'urrent -is nearly zero? Considering now the" current density iiiia conductor bar of a secondary windingywhen' the frequency is Zero the fc'qrr'ent' 'distribul i qi i jjthf esh t th c de As the frequency increases, the currentdehsityjfoff top. "of "the conductor increases 'while'theburrent density at. the bottom ,of

the pond a a sesses dlie eas est that thefreactaiice is higher at theft) t to'm of the conductor than at: the top. Atfir st with 'relativelyl 'low frequency, the; react-ance' is Small i pmperi d iw h h"'ehn i esi tance of thefeondu c't or, which "accounts for sulch lossesj'jare greatest "when" the or tbeii'igfreversed t When the ,filotor [the verylgrad ial change A in the current dis- For a better understanding gg in'vention, referenceis'madeiinthefollowing, de- 1 I 1 V Consideringjnowthe flux distribution, the

scri tion 'to: the accom an iii 'fdrawin p .a a:

where I have, shownin Fig. a sguirrel T so as tocause equal secondary currents to der such conditions; the'frecomes in 'coinparison with the :ohmic rsis tahlfe ';a;nd' ,the"ehange in current distribution i's the quite rapid.

' due to the fact that thedensity' of thejflux at theQtop of the conductor is determined by the total current flowing in the'con'ductor,

whereas the density of, the flux which cuts the conductor at any point radially beneath its top is determined by the fraction of the total current flowing in that section of the conductor located beneath the point of reference. Thus, the density of the tin; which cuts the conductor at any point radially beucath the top of the conductor decreases as the frequency increases due to the crowding of the current toward the top of the conductor.

These principles apply alike to the winding described in the Kierstead patent previrinsly referred to, and to the winding of the present invention. The" present invention is limited to so proportioning the depths of the top and bottom conductors as to increase the etl'cctivcness, for a given available total depth, of a secondary winding involving these principles.

Referring now to the drawing, Fig. I shows a squirrel cage winding made up of end rings 10 and it and composite bars 12 having sections 13 and 14. Each section has a relatively deep portion at one end and a relatively shallow portion at the other end and the sections are shaped and recessed at the centers as shown at 15, Figs 3 and 4, so as to cross each other from top to bottom in the slot and when thus titted together, as represented in Fig. 2, have a shape substantially conforming to a straight rectangular bar and are thus adapted to fit, into standard rectangular slots. The contacting surfaces of the two sections are preferably enainelled or coated with shellac or somoother insulating compound so that the secondary current flowing from end to end through the bais is substantially equal in the two seetions.

It will be evident that at the instant of starting, when the secondary currentis a maximum, there will also be considerable eddy current-in the upper section which does not exist, except to a very limited extent, in the lower section. These eddy currents p ro duee heating losses and starting torque. Consequently, the heat loss through the component conductor bars at starting will be more evenly distributed than would be the case if both the upper and lower sections were of the same cross section and depth. When the motor comes up to speed andthe eddy currents decrease to a negligible value, the normal secondary current also decreases, although not to the same extent, so that the limiting factor so far as heating is concerned, is larg zely a matter of care of the heat loss at starting. For normal operation, the current density in the lower section may be quite high because of the relatively good thermal contact which such section has with the larger upper section. Consequently, the heat is rapidly d ssipated from the lower conductor sections.

, that threw? i Referrin .now to Fig. 9, I have diagrammatically il ustrated in the left hand column the current distribution in the composite eonduetor of my invention when the current is (I) of zero frequency, (II) of relatively low 7 hand column indicate that part of the total flux which is due to the current flowing in the lower section 14 of the composite conductor. It will be evident that the flux distribution through the upper conductor section is substantially what it would be if the lower conductor section were. made of the same depth and cross sectional area as the uppersection. Consequently, it will be evidentthat the advantages of the windings described ill-I110 Kie stead patent are retained by this construction, but that the total depth of slot neeessaryto obtain these advantages has been materially reducedby my invention, or for a-given total depth of slot, a greater loss and more torque at starting are obtained. i

In Fig. 8 I have represented an alteruativ,e arrangement-where the-lower copper .seetioneomprises a.' round bar instead of being rectangular. V A slightly different cur- 3 rentfdis tribution at'high' frequency will be produced in the lower conduetor by this medi catioa, but th flaxdis r o w maiasuhst nt a ly t e Same a p n e n afi; 15 n.

In accordance with the"provisions 'of the e s-H sta e are d s r b dth D w th the a pa a s whi h Ilimw sena represent ehesternbodiment thereof, but I desireto have it understood that the. appar tu h wn and cri isp'ntv n strative and-thatthe invention maybe carri si v out by-ether mea that I claim as new and desire to secure y L wli 't n oi the d State In a littlest-i s moto Ia c n ao' m mb e n-P ssing magnetic mem er-h i g aelerelltrc re ati ely e p J Istadi y arrange e qt r ats in eac lot, l sh uc r. ar bein e. e m

lialtits length it is a bottom bar of re yely srn'al'l de .t-h and through the remainder of itslengt I it is atop bar of relatively lame depth,'the conductor bars in the sa ne slot connected at their ends to amm n end n witn'esswhereof, I have hereunto setrny hand this 3rd day of October, 1924.

' I i i HENRY lVl. IIOBART.

Ell) 

